1. Field of the Invention
The present invention relates to an alarm device for informing reduction of pneumatic pressure of vehicle tires by sensing abnormal change of tire internal pressure of a vehicle when running and by informing it to an operator.
2. Description of the Prior Art
An alarm device for informing an alarm to an operator by sensing abnormal change caused by reduced pressure of internal pressure of a tire in a vehicle, such as a puncture and the like, is well known by for example U.S. Pat. No. 3,810,090. Such device comprises a transmitter having an oscillator for generating a high frequency signal by sensing change of tire internal pressure and a transmitter antenna for radiating an electromagnetic wave by its output, a receiver having a receiver antenna attached on the chassis side of a vehicle for receiving the electromagnetic wave from the transmitter antenna and for processing the electromagnetic wave received by this antenna, and an alarm for generating a warning to an operator.
This kind of devices, however, is liable to be misoperated by receiving an influence of noises from a spark plug generated by a vehicle itself and noises from the outside and lacks reliability. In order to construct a device for receiving no influence of such noises, its mechanism becomes complicated and as a result, the device becomes expensive and impracticable.